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Use a grout saw to remove stained or damaged grout; then flush the area with water.
Mix the grout or use a premixed grout. If necessary, add colorant to make it the correct color.
Tip: Be sure to check the label when buying grout -- you need to have the correct type for your tile. For marble or shiny tile, don't use sanded grout -- it's just like using sandpaper, the rough texture can damage the tiles. If you're not using pre-mixed grout, wear cloves and use your hands to combine the powdered grout with water.
With a grout float -- or, if you're working on a small area, a sponge -- apply the new grout to the tiles. Don't worry if it gets on the tile -- you'll use the sponge to remove the excess later. Once the grout has been applied and has cured according to label directions, be sure to use a grout sealer.
Tip: If your grout is merely discolored, you may be able to get away with just using a grout colorant instead of re-grouting.